Sumter was an early manufacturer of low tension ignition equipment for the burgeoning farm engine market. Sumter was later acquired by Splitdorf. Their products were common on the early Fairbanks Morse Z engines. Low tension ignition systems operated on much lower voltages than spark ignition systems and worked by opening a pair of electrical contacts(the Ignitor) connected across the output of the low tension magneto or battery in series with a low tension coil inside the combustion chamber creating a small spark when the contacts separated. Contacts inside the cylinder required constant cleaning and the mechanical moving parts are often found well worn and/or rusted solid.
With a low tension magneto,essentailly an alternator, the output is an AC sine wave. In one rotation of the magneto shaft the output voltage rises from zero volts to a positive peak voltage then from the positive peak to zero volts then to a negative peak and finally returning to zero volts. To get the biggest bang out of the low tension magneto we must open separate the ignitor contacts when the magneto output is at a peak be it positive or negative. That is why most low tension magnetos have witness marks for timing. Ultimately we want the ignitor to snap open at or slightly before top center when the magneto is at maximum output. Refer to the engine builder for exact procedures.Low tension magnetos come in a variety of configurations as rotating and oscillating, but all work on similar principles. Some engines started with a battery and coil then switched to a low tension magneto. It is important to never feed voltage into a magneto. This can demagnetize the magnet and possibly damage the coil.
Here is some factory service information to keep yours alive and well.
Timing as described on the models 10, 11, and 12 is how the Fairbanks Morse Z engines are set up. The witness marks are usually on the end of the magneto shaft and bearing.
The procedure described above is how we get the peak output of the mag when the points open. Some of the later models have two witness marks on the bearing or endcap and the witness mark on the shaft is located between them.
Notice how the battery is configured so that once the engine gets rolling in the battery mode,it is witched out of the circuit in the magneto mode. Feeding battery current into the magneto will discharge the magnets and possibly damage the coil.